Chromium enterobactin

The resolution of tris(catecholato)chromate(III) has been achieved by crystallization with L-[Co(en)3]3+ the diastereomeric salt isolated contained the L-[Cr(cat)3]3 ion.793 Comparison of the properties of this anion with the chromium(III) enterobactin complex suggested that the natural product stereospeeifically forms the L-cis complex with chromium(III) (190). The tris(catecholate) complex K3[Cr(Cat)3]-5H20 crystallizes in space group C2/c with a = 20.796, 6 = 15.847 and c = 12.273 A and jS = 91.84° the chelate rings are planar.794 Electrochemical and spectroscopic studies of this complex have also been undertaken.795 Recent molecular orbital calculations796 on quinone complexes are consistent with the ligand-centred redox chemistry generally proposed for these systems.788... 

Raymond and co-workers have synthesized and separated optical and geometrical isomers of simple tris hydroxamate chroniium(III) and tris phenolate chromium(III) or rhodium(III) complexes and assigned the absolute configurations of these isomers based on criteria such as chromatographic behavior due to differences in dipole moment, theoretical symmetry considerations, and X-ray crystallographic data 174). The absolute configuration of isomers of chromium(III) complexes of ferrichrome, ferrichrysin 175), ferrioxamine B and Dx 176), rhodotorulic acid 177), enterobactin 178),... 

The simple model complex, tris(cateeholato)chromate(III) has been prepared, and complete resolution of the optical isomers was achieved at pH 13. The known crystal structure of [Cr(cat)3]3- and arguments similar to those for the hydroxamate chromium complexes lead to the assignments of absolute configuration of the CD spectra. It was found that the CD spectra of A-[Cr(cat)3f and [Cr(ent)3f " are essentially identical, and the mirror image of chromic desferriferrichrome (Fig. 28), which shows that enterobactin has a predominant A-cis absolute configuration 147). Unfortunately the usual oxidation sensitivity of the catechol dianion is substantially increased in the chromium complexes, which precludes their use as biological probes l47). 

For the ferric siderophore complexes, comparison of the CD spectra of the chromium complexes of ferrichrome and enterobactin with the CD spectra of their iron complexes [and the separation of optical isomers of even ferric(benzhydroxamate)3 complexes in nonaqueous solution 192)] have shown that the same rule applied to the CD spectra for chromium complexes can be used for iron siderophore complexes as well iron(III) complexes will have a predominant A configuration in solution if the CD band in... 

In addition to preparing the model catecholate complexes of rhodium and chromium, the analogous enterobactin complexes were also prepared and their CD spectra recorded (62). From examination of molecular models it is apparent that either the A-cis or A-cis diastereomers of a metal enterobactin complex are structurally possible. In theory, these diastereomers should be separable by chromatographic techniques analogous to those used for the hydroxamates (vide supra) however, under a variety of conditions only one chromatographic fraction is obtained. We conclude that one isomer predominates to the exclusion of the other. 

The complex [CrLs] where H2L = l,2-(HO)2QH4 is a model complex for enterobactin. How is the model related to enterobactin, and what is the reason for chromium-for-iron substitution ... 

Table 2 UV/vis and CD spectra in solution of various geometrical and optical isomers of chromium (III) siderophores and of model compounds rhodoturulic acid (A), men (B), coprogen (C), enterobactin (D), catechol (E), and ferrioxamine B (F). UV/vis values are for Xmax (nm) and e (M cm ) CD values are for X ax (nm) and 0 (M cm )...

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